1. Field of the Invention
This invention relates to an antistatic cathode ray tube having the effect of preventing electric charging by static induction on the front panel, and having a high degree of glossiness, excellent resolution and contrast characteristics, and high scratch resistance.
2. Related Art
When a cathode ray tube is set in a television receiver and operated, the front panel surface of the tube is electrically charged due to static induction which is caused by switching on and off of power or application of high voltage in the operation. This static electricity attracts dust and other contaminants in the air and collects them on the panel surface to foul it, making it difficult to view the picture images displayed on the screen. Also, when a person touches the panel surface, he may have an unpleasant feeling or a slight shock by discharge of static electricity.
As a method for preventing such static charging of the front panel of cathode ray tube, it has been proposed to provide a transparent electroconductive film layer on the external surface of the panel (see, for instance, Japanese Patent Application Kokai (Laid-Open) No. 16452/86, 124331/88 and 160140/88). Various methods have been proposed for forming a transparent electroconductive film layer of this type. According to one method, a solution containing an electroconductive material is spray-coated on the external surface of the panel to provide it with both an anti-glaring effect and an antistatic effect. In another method, a solution containing an electroconductive material is spin-coated or dip-coated on the panel surface.
For example, Japanese Patent Application Kokai (Laid-Open) No. 124331/99 discloses a method according to which a solution prepared by adding one or more of the compounds selected from halides, nitrates and sulfates of Fe, Co, Ni, Ag, Zn, Al and In to a silica sol solution made by using silicon halide, silane alkoxide, silyl isocyanate or the like is spray-coated on the panel surface of a cathode ray tube and then baked. According to this method, the front surface of cathode ray tube is provided with excellent antistatic properties and also has high film strength, but the surface resistivity provided by this treatment is around 19.sup.9 .OMEGA./.quadrature. and also glossiness is only about 50%, which leads to low resolution and constrast, so that the thus treated panel surface is impractical where super-fineness of pictures is an important requirement. For instance, it is unsuited for a cathode ray tube for display devices for OA appliances where a glare type panel is required.
A glare type transparent electroconductive film of an anti-static cathode ray tube can be obtained by spin-coating or dip-coating a solution containing an electroconductive material on the panel surface; but in case of using these methods, particular care is needed for ensuring cleanliness of the panel surface because presence of contaminants such as dust on the panel surface at the time of coating causes a serious defect of the coat. Also, even a slight change of coating conditions may cause formation of interference color or non-uniform coloration, resulting in a low yield of products.
As means for increasing the strength of the transparent electroconductive film, a method is known in which a solution prepared by dispersing ultra-fine particles mainly composed of electroconductive particles of tin oxide (SnO.sub.2) in an alcoholic solution such as ethyl silicate solution, is coated on the surface of the object to be coated and then baked at a relatively high temperature, or about 500.degree. C., to form a transparent electroconductive coating film. However, use of such a high baking temperature as about 500.degree. C., although capable of providing a film having practically sufficient mechanical and chemical strength, is improper for application to a finished product such as a cathode ray tube or a liquid crystal display element. For a finished product, the baking temperature must not exceed 200.degree. C.
It is, however, impossible to obtain a high film strength with such a low baking temperature. As a solution to this problem, it has been proposed to apply an additional surface protective coating film on the transparent electroconductive film. This protective film is formed by spray coating, and it is quite satisfactory in strength and also has a reflection preventing function.
By forming a surface protective film as described above, it is possible to obtain a high film strength and also the production yield is enhanced; but in this case, too, the panel surface of the cathode ray tube is lowered in glossiness, resulting in reduced resolution and constrast. When it is attempted to raise surface glossiness to around 80% by reducing the spray-coating rate, it becomes impossible to maintain the practical film strength. Also, there is a large possibility of causing formation of interference color or non-uniformity of coloration due to the two-layer structure, and it is also difficult to attain a high-yield production.
In the case of spin-coating where said protective film is formed in a more moist state, although resolution and clearness of image are improved, there occurs a phenomenon of interference between the protective film and the base film to cause coloration.
The tests on glossiness, resolution and film strength by the present inventors have revealed the following facts.
Firstly, the inventors examined the relation between glossiness and resolution according to the Bar-chart method by using a cathode ray tube having a transparent electroconductive layer and a protective film layer formed on said electroconductive layer. The electroconductive layer being formed by spin-coating an alcohol solution of tin oxide (SnO.sub.2), antimony oxide (Sb.sub.2 O.sub.3) and alkyl silicate on the surface of a tinted bulb (transmittance of light with wavelength of 546 nm at 10 mm thickness=56%) and the protective film layer being formed by spray-coating on the electroconductive layer an alcohol solution of alkyl silicate. The result is shown in FIG. 6. It is seen from the graph of FIG. 6 that glossiness (measured according to JIS Z-8741 Method 2) in the case of providing a transparent electroconductive layer alone (providing no protective layer) is 110% (indicated by a white circle in the graph), which dictates that it is necessary to make glossiness 80% or higher for obtaining a same degree of resolution when providing a protective layer.
FIG. 7 shows the result of examination on the relation between glossiness and pencil scratch hardness (measured according to JIS K-5401) by using the same specimen as employed in the above test. The result of examination depicted in the graph of FIG. 7 shows that when it is attempted to provide the film with a pencil hardness of H or above which is the practical strength of film, glossiness becomes lower than 80%.